The economics of Neospora caninum control, using calf serology, in two New Zealand dairy herds.

AIM: To determine the cost-benefit of a control programme that used serological testing of young heifer replacement calves to reduce Neospora caninum seroprevalence in two dairy herds.

METHODS

Data were collected between 2016 and 2023 from replacement heifer calf cohorts that were tested at a young age for N. caninum antibodies, on two adjacent Waikato dairy farms with endemic neosporosis. Calf serology (ELISA) results were interpreted using cut-off values adjusted to compensate for the presence of maternal antibodies, to classify calves as seronegative, inconclusive, or seropositive. Serological testing costs to establish the N. caninum status of calves were obtained alongside rearing costs from birth up to the point at which the calves' serostatus was established or, for serologically negative animals, to 24 months of age when replacement heifers are expected to produce their first calf. A formula was developed to calculate a break-even point: when calf testing costs were equal to the financial benefit of not rearing unwanted seropositive replacement heifers. The calculation did not include any potential lifetime costs that could be attributed to abortion or decreased production, but is valid when the farmer's aim is to avoid rearing N. caninum seropositive replacement calves and to eliminate the disease from the herd over time.

RESULTS

Testing young heifer replacement calves with an N. caninum antibody ELISA to identify congenitally infected calves was of economic benefit when the proportion of seropositive replacement calves was > 2.87%. A sensitivity analysis showed that the cost of heifer rearing had the greatest impact on the break-even point, followed by the cost of the initial test.

CONCLUSION AND CLINICAL RELEVANCE: As the proportion of N. caninum-infected cows in the herd reduces and the proportion of infected calves drops below 3%, young-calf testing is no longer cost-effective, and targeted testing becomes a more economical alternative. For example, monitoring for vertical transmission if valuable seropositive cows are mated to produce a potential replacement calf and checking for horizontal infection by retesting animals that had seronegative results as young calves. When the herd seroprevalence is low, horizontal infection becomes the main route of infection, and biosecurity should be maintained at a high standard. Consideration should also be given to the limitations of the diagnostic tests that are used for monitoring, as these have been developed for adult cattle that have recently aborted due to N. caninum rather than persistently infected animals.